In cellular systems such as orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) wireless systems, an access channel is commonly used for a mobile station (MS) to request access and acquire ownership of an uplink data channel in order to initiate transmission with a base station (BS). In current design of IEEE 802.16e, there are four types of access channels for the MS to send an access request: an initial ranging channel, a handover (HO) ranging channel, a bandwidth request (BW-REQ) ranging channel, and a periodic ranging channel. An access request procedure usually consists of two phases: a contention resolution phase and a request negotiation phase. Take an IEEE 802.16e BW-REQ ranging procedure as an example, in the first phase, the MS sends a BW-REQ ranging code via a shared channel, and the BS acknowledges after detecting which mobile station sends the BW-REQ ranging code. In the second phase, the MS sends a BW-REQ message for bandwidth allocation, and the BS grants uplink resource after correctly decoding the BW-REQ message. After successful contention and negotiation in both phases, the MS is then able to start scheduled uplink transmission. Therefore, it takes five steps and is quite time-consuming to complete the entire BW-REQ ranging procedure.
FIG. 1 (Prior Art) illustrates sequence charts of a 3-step BW-REQ ranging procedure and a fallback 5-step BW-REQ ranging procedure. In a 3-step BW-REQ ranging procedure illustrated in the left side of FIG. 1, the MS first sends a BW-REQ ranging code with an embedded BW-REQ message. The BS detects the BW-REQ ranging code, decodes the BW-REQ message, and grants uplink resource. The MS then starts scheduled uplink data transmission. By sending the BW-REQ ranging code and the BW-REQ message together, BW-REQ ranging latency is shortened. This 3-step BW-REQ ranging procedure, however, falls back to a 5-step BW-REQ ranging procedure when ranging collision occurs. As illustrated in the right side of FIG. 1, the MS first sends a BW-REQ ranging code with an embedded BW-REQ message. Due to multiple ranging transmission from multiple mobile stations (i.e., multiple mobile stations send BW-REQ with different BW-REQ ranging codes and messages at the same time), the BW-REQ messages may not be decodable by the BS while the BW-REQ ranging codes are decodable. This is because BW-REQ ranging code is usually more robust than BW-REQ message in the BW-REQ ranging design. As a result, the MS retransmits the BW-REQ message to the BS. The BS decodes the BW-REQ message successfully and grants uplink resource. Finally, the MS is able to start scheduled uplink data transmission. Therefore, in the fallback 5-step BW-REQ ranging procedure, the access latency is not reduced.
FIG. 2 (Prior Art) illustrates a physical (PHY) layer channel structure of a resource block 20 used for BW-REQ ranging channel transmission. Resource block 20 is a two-dimensional 18×6 BW-REQ channel, allocated into three distributed 6×6 BW-REQ tiles. Each BW-REQ tile comprises six time slots (OFDM symbol) in the time domain and six frequency tones (subcarrier) in the frequency domain. In the example of FIG. 2, to facilitate the 3-step BW-REQ ranging procedure illustrated in FIG. 1, the BW-REQ ranging code is robustly designed as preambles for reliable non-coherent detection, while the BW-REQ message is less robust for coherent detection.
Although the 3-step BR ranging procedure illustrated in FIG. 1 and the PHY layer BW-REQ channel structure illustrated in FIG. 2 are designed to shorten access latency, such design is based on single-antenna MS operation. Therefore, the design does not exploit transmission gains of multiple antennas such as diversity gain and beamforming gain that can be achieved by multiple-input multiple-output (MIMO) techniques. As a result, due to the large performance gap between BW-REQ ranging code and BW-REQ message, the design may face high probability of falling back to 5-step BW-REQ ranging procedure and thus results in no latency improvement and the waste of radio resources in BW-REQ message. For example, if multiple MSs transmit BW-REQ with different BW-REQ ranging codes and messages to the BS at the same time, then some or all the MSs may have to retransmit their BW-REQ messages and fall back to 5-step BW-REQ ranging procedure because the BS may not be able to decode some or any of the BW-REQ messages successfully. A solution is sought for further performance improvement.